Cap the number of patterns pointed to by the lint

compiler/rustc_mir_build/messages.ftl

@@ -327,6 +327,8 @@ mir_build_union_pattern = cannot use unions in constant patterns
 
 mir_build_unreachable_making_this_unreachable = collectively making this unreachable
 
+mir_build_unreachable_making_this_unreachable_n_more = ...and {$covered_by_many_n_more_count} other patterns collectively make this unreachable
+
 mir_build_unreachable_matches_same_values = matches some of the same values
 
 mir_build_unreachable_pattern = unreachable pattern

compiler/rustc_mir_build/src/errors.rs

@@ -596,6 +596,7 @@ pub(crate) struct UnreachablePattern<'tcx> {
     pub(crate) covered_by_one: Option<Span>,
     #[note(mir_build_unreachable_covered_by_many)]
     pub(crate) covered_by_many: Option<MultiSpan>,
+    pub(crate) covered_by_many_n_more_count: usize,
 }
 
 #[derive(Subdiagnostic)]

compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -917,6 +917,7 @@ fn report_unreachable_pattern<'p, 'tcx>(
     pat: &DeconstructedPat<'p, 'tcx>,
     explanation: &RedundancyExplanation<'p, 'tcx>,
 ) {
+    static CAP_COVERED_BY_MANY: usize = 4;
     let pat_span = pat.data().span;
     let mut lint = UnreachablePattern {
         span: Some(pat_span),
@@ -925,6 +926,7 @@ fn report_unreachable_pattern<'p, 'tcx>(
         covered_by_catchall: None,
         covered_by_one: None,
         covered_by_many: None,
+        covered_by_many_n_more_count: 0,
     };
     match explanation.covered_by.as_slice() {
         [] => {
@@ -950,15 +952,27 @@ fn report_unreachable_pattern<'p, 'tcx>(
             lint.covered_by_one = Some(covering_pat.data().span);
         }
         covering_pats => {
+            let mut iter = covering_pats.iter();
             let mut multispan = MultiSpan::from_span(pat_span);
-            for p in covering_pats {
+            for p in iter.by_ref().take(CAP_COVERED_BY_MANY) {
                 multispan.push_span_label(
                     p.data().span,
                     fluent::mir_build_unreachable_matches_same_values,
                 );
             }
-            multispan
+            let remain = iter.count();
-                .push_span_label(pat_span, fluent::mir_build_unreachable_making_this_unreachable);
+            if remain == 0 {
+                multispan.push_span_label(
+                    pat_span,
+                    fluent::mir_build_unreachable_making_this_unreachable,
+                );
+            } else {
+                lint.covered_by_many_n_more_count = remain;
+                multispan.push_span_label(
+                    pat_span,
+                    fluent::mir_build_unreachable_making_this_unreachable_n_more,
+                );
+            }
             lint.covered_by_many = Some(multispan);
         }
     }

tests/ui/pattern/usefulness/explain-unreachable-pats.rs

@@ -26,6 +26,25 @@ fn main() {
         _ => {}
     }
 
+    match 0u8 {
+        1 => {}
+        //~^ NOTE matches some of the same values
+        2 => {}
+        //~^ NOTE matches some of the same values
+        3 => {}
+        //~^ NOTE matches some of the same values
+        4 => {}
+        //~^ NOTE matches some of the same values
+        5 => {}
+        6 => {}
+        1 ..= 6 => {}
+        //~^ ERROR unreachable pattern
+        //~| NOTE no value can reach this
+        //~| NOTE multiple earlier patterns match some of the same values
+        //~| NOTE ...and 2 other patterns
+        _ => {}
+    }
+
     let res: Result<(),!> = Ok(());
     match res {
         Ok(_) => {}

tests/ui/pattern/usefulness/explain-unreachable-pats.stderr

@@ -32,7 +32,31 @@ LL |         (1 | 2,) => {}
    |         ^^^^^^^^ collectively making this unreachable
 
 error: unreachable pattern
-  --> $DIR/explain-unreachable-pats.rs:32:9
+  --> $DIR/explain-unreachable-pats.rs:40:9
+   |
+LL |         1 ..= 6 => {}
+   |         ^^^^^^^ no value can reach this
+   |
+note: multiple earlier patterns match some of the same values
+  --> $DIR/explain-unreachable-pats.rs:40:9
+   |
+LL |         1 => {}
+   |         - matches some of the same values
+LL |
+LL |         2 => {}
+   |         - matches some of the same values
+LL |
+LL |         3 => {}
+   |         - matches some of the same values
+LL |
+LL |         4 => {}
+   |         - matches some of the same values
+...
+LL |         1 ..= 6 => {}
+   |         ^^^^^^^ ...and 2 other patterns collectively make this unreachable
+
+error: unreachable pattern
+  --> $DIR/explain-unreachable-pats.rs:51:9
    |
 LL |         Err(_) => {}
    |         ^^^^^^
@@ -41,7 +65,7 @@ LL |         Err(_) => {}
    = note: to learn more about uninhabited types, see https://doc.rust-lang.org/nomicon/exotic-sizes.html#empty-types
 
 error: unreachable pattern
-  --> $DIR/explain-unreachable-pats.rs:46:9
+  --> $DIR/explain-unreachable-pats.rs:65:9
    |
 LL |         (Err(_), Err(_)) => {}
    |         ^^^^^^^^^^^^^^^^
@@ -50,7 +74,7 @@ LL |         (Err(_), Err(_)) => {}
    = note: to learn more about uninhabited types, see https://doc.rust-lang.org/nomicon/exotic-sizes.html#empty-types
 
 error: unreachable pattern
-  --> $DIR/explain-unreachable-pats.rs:53:9
+  --> $DIR/explain-unreachable-pats.rs:72:9
    |
 LL |         (Err(_), Err(_)) => {}
    |         ^^^^^^^^^^^^^^^^
@@ -59,7 +83,7 @@ LL |         (Err(_), Err(_)) => {}
    = note: to learn more about uninhabited types, see https://doc.rust-lang.org/nomicon/exotic-sizes.html#empty-types
 
 error: unreachable pattern
-  --> $DIR/explain-unreachable-pats.rs:63:11
+  --> $DIR/explain-unreachable-pats.rs:82:11
    |
 LL |     if let (0
    |             - matches all the relevant values
@@ -68,13 +92,13 @@ LL |         | 0, _) = (0, 0) {}
    |           ^ no value can reach this
 
 error: unreachable pattern
-  --> $DIR/explain-unreachable-pats.rs:73:9
+  --> $DIR/explain-unreachable-pats.rs:92:9
    |
 LL |         (_, true) => {}
    |         ^^^^^^^^^ no value can reach this
    |
 note: multiple earlier patterns match some of the same values
-  --> $DIR/explain-unreachable-pats.rs:73:9
+  --> $DIR/explain-unreachable-pats.rs:92:9
    |
 LL |         (true, _) => {}
    |         --------- matches some of the same values
@@ -86,7 +110,7 @@ LL |         (_, true) => {}
    |         ^^^^^^^^^ collectively making this unreachable
 
 error: unreachable pattern
-  --> $DIR/explain-unreachable-pats.rs:86:9
+  --> $DIR/explain-unreachable-pats.rs:105:9
    |
 LL |         (true, _) => {}
    |         --------- matches all the relevant values
@@ -95,7 +119,7 @@ LL |         (true, true) => {}
    |         ^^^^^^^^^^^^ no value can reach this
 
 error: unreachable pattern
-  --> $DIR/explain-unreachable-pats.rs:98:9
+  --> $DIR/explain-unreachable-pats.rs:117:9
    |
 LL |         (_, true, 0..10) => {}
    |         ---------------- matches all the relevant values
@@ -103,5 +127,5 @@ LL |         (_, true, 0..10) => {}
 LL |         (_, true, 3) => {}
    |         ^^^^^^^^^^^^ no value can reach this
 
-error: aborting due to 9 previous errors
+error: aborting due to 10 previous errors